Protective metal coatings are commonly applied to metal surfaces to improve corrosion resistance and paint adherence characteristics. These protective metal coatings are referred to as conversion coatings and consist of a variety of protective treatments including iron phosphate, manganese phosphate, zinc phosphate, zinc phosphate modified with calcium, nickel, or magnesium ions, mixed metal oxides and titanium or zirconium organometallic coatings. These protective treatments may be applied to a multiplicity of different metals such as zinc, iron, aluminum, and cold-rolled, ground, pickled, and hot-rolled steel and galvanized steel surfaces. As used herein, "metal surface" shall include both untreated metal surfaces and those to which a conversion coating has been applied.
Normally, after a conversion coating has been applied to the requisite metal surface, the surface is subjected to a final rinsing step to enhance the corrosion resistance and to prepare the surface for the reception of a final finish coating, such as a paint, enamel or Japan varnish.
Traditionally, hexavalent and trivalent chromium rinse solutions have been used for this post-conversion coating rinse process. Unfortunately, such chromium compounds have fallen into disfavor due to their inherent toxicity problems and the ensuing problem of waste disposal due to the presence of hexavalent and/or trivalent chromium in process effluents.
Another problem that needs to be mentioned, in conjunction with such chromium-based post-conversion coating rinses, is that certain paint types, when applied to chromium-treated metal surfaces, tend to chip, peel, and/or blister. Additionally, certain surfaces with non-planar contours tend to accumulate residues of chromium salts thus exacerbating the aforementioned peeling and blistering problems.
In an attempt to move away from the chromium-based rinse systems, amine, tannin, titanium, zinc, zirconium, and aminoalkylated polyvinylalcohol rinses have been attempted. However, these chromium-free post-conversion coating rinse programs have not earned wide acceptance, principally due to disappointing performance in retarding corrosion.